Optimizing HV Lightning Impulse Tests for Power Transformers

Lightning Impulse testing is a fundamental type test for power transformers, designed to prove the strength of their insulation against high-voltage transients caused by atmospheric lightning strikes. Compliance with standards such as IEC 60076-3 and IEEE Std C57.98 is mandatory for manufacturers and testing laboratories. A successful test campaign hinges on meticulous procedure, precise setup, and understanding of waveform nuances.

Standard Test Waveforms and Sequence

The standard full lightning impulse is defined as a 1.2/50 µs wave. The test sequence for a transformer winding typically requires:
1. One reduced full wave impulse at between 50% to 75% of the full test voltage.
2. Two consecutive full wave impulses at 100% of the rated Basic Impulse Level (BIL).
3. Several chopped wave impulses, where the wave is chopped on the tail, typically after 2-6 µs. The chopping gap must be adjusted to achieve a chop time between 0.5 and 2.0 µs from the virtual origin.

Critical Test Setup Considerations

The physical setup is paramount. The impulse generator must be properly grounded to a low-impedance earth grid to prevent dangerous potential rise. The test object (transformer) should be isolated from its foundation using insulating supports. All bushing covers and non-test terminals must be properly connected: line terminals to the generator, tank and core grounded, and other windings short-circuited and connected to ground through a measuring shunt for induced current recording (the transferred surge test).

Measurement and Failure Detection

Failure detection is primarily achieved through comparative wave analysis. The digital recordings of the applied voltage wave and the neutral current wave (or a winding current) from the reduced impulse are compared with those from the full-voltage impulses. Any significant deviation in shape, oscillation pattern, or timing indicates a potential insulation breakdown. Using high-bandwidth digital recorders and ensuring identical cable lengths for all measurement channels are essential for valid comparison.

Mitigating Common Challenges

Several challenges can complicate transformer impulse testing. Low-frequency oscillations on the wavefront can be mitigated by adding damping resistors or adjusting the generator's front resistance. High ringing frequencies are often addressed by minimizing lead inductance through tight, direct connections. For large transformers with high capacitance, the generator's energy rating must be sufficient to maintain the proper wavetail without excessive droop, sometimes requiring external tail capacitors.

Conclusion: Ensuring Valid and Safe Tests

Executing a valid lightning impulse test requires more than just a capable generator. It demands a rigorous adherence to standardized procedures, a deep understanding of the test object's characteristics, and a systematic approach to setup and measurement. By following these best practices, testing engineers can ensure reliable results that conclusively demonstrate a transformer's insulation integrity, ultimately contributing to the safety and reliability of the power grid.